1. Field of the Invention
This invention relates to a short-range Doppler radar; and more particularly, it relates to a Doppler radar system of the type which may be carried by a vehicle, such as an automobile, or any vehicle in a broad sense so as to measure the velocity thereof by radiating a microwave signal and detecting the Doppler frequency shift undergone by the reflected signal. Especially, it relates to the construction of a transmitting and receiving device for microwave signals.
2. Description of the Prior Art
In short-range radars, such as a Doppler radar system for measuring the true ground speed of an automobile, the transmission power should be kept to a very low power level (1-10 .mu.W or less) from the viewpoint of the possible influence of such signal on other radio equipment which may be within the range of the radar signal. On the other hand, the receiving sensitivity of the system ought to be made as high as possible to ensure a high measuring accuracy.
A Doppler radar utilizing microwave signals is so constructed that in order to detect the Doppler frequency shift of received reflected waves, a mixing circuit which mixes the received waves and local oscillation waves is provided so as to detect the difference component of the two frequencies from the output thereof. In order to make a device for use in an automobile as small in size as possible, a transmitter/receiver for the microwave signal is constructed of a microwave integrated circuit, and a mixer diode is used as the mixing circuit.
Usually, in order to operate the mixer diode at a high sensitivity, approximately 1 mW needs to be applied as a driving power. In this case, a special balanced mixer or the like comprising a pair of diodes may be employed for the purpose of reducing the microwave leakage power. However, there has been the problem that when the characteristics of the two diodes which constitute the balanced mixer degrade at an unequal rate, the transmission power varies sharply.